Refrigeration systems are widely used for providing and maintaining controlled and cryogenic temperatures for various applications. Among other examples, well known are cooled Infrared (IR) imagers, converting infrared radiation into visual imagery. Generally, the operating principle of IR imagers is based on the fact that warmer objects radiate in IR wavelength range more, and colder object radiate less. Noise levels in IR detection are usually strongly dependent on the operating temperature of the IR detector and therefore, high-edge imagers generally rely on mechanical Stirling cryogenic cooling.
Stirling cryocoolers, which may be of both split and integral types, typically comprise two major components: a compressor and an expander. In a split cooler these are interconnected by a flexible gas transfer line (a thin-walled stainless steel tube of a small diameter) to provide for maximum flexibility in the system design and to isolate the IR detector from the vibration interference which is produced by the compressor. In the integral cooler these components are integrated in a common casing.
The reciprocating motion of a compressor piston provides the required pressure pulses and the volumetric reciprocal change of a working agent (helium, typically) in the expansion space of an expander. A displacer, reciprocating inside a cold finger, shuttles the working agent back and forth from the cold side to the warm side of the cooler. During the expansion stage of the thermodynamic cycle, heat is absorbed from the IR detector mounted upon the cold finger tip (cold side of a cycle), and during the compression stage, heat is rejected to the ambient from the cold finger base (warm side of a cycle).
Modern split Stirling linear cryocoolers are usually comparable with their rotary rivals in terms of weight, size and power consumption. However they offer higher reliability, lower noise and vibration signature over the typical high frequency range, aural non-detectability, lower parasitic losses, flexibility in the system design, etc.
As different from fully counterbalanced integral rotary rivals, the vibration export produced by the separated compressor and expander units of a typical split Stirling cryocooler is normally higher at the driving frequency. This especially holds true for cryocoolers featuring single piston imbalanced compressors. Using dual-piston compressors results in particular attenuation of vibration export, however, because of the sub-compressor dissimilarity, not to the extent typical of integral rotary cryocoolers. In spite of the small moving mass and stroke of the fundamentally single piston unbalanced expander, its vibration export cannot be ignored.
Typically, the compressor and expander units are placed side by side with minimum offset, thus forming the most compact space saving U shape. In general, vibration export produced by such a split linear cooler may be characterized as a combination of two dynamic forces or as a resultant dynamic force and moment, the frequency of which equals the driving frequency. Depending on the payload inertia and mounting conditions, relative distance between the compressor and expander units along with the distance from the payload center of gravity, this vibration export may result in translation and angular vibration manifesting itself in dynamic defocusing occurring when translational displacement becomes comparable with the focus depth and line of sight jitter occurring when the amplitude of in-plane focal plane array (FPA) motion becomes comparable with pixel size.
Tuned dynamic absorbers (TDA) are widely used for attenuating the vibration export produced by the split Stirling cryogenic coolers, typically working at constant driving frequency.
U.S. Pat. No. 5,895,033 describes a passive balance system for counterbalancing vibrations of a machine. The passive balance system includes a support member adapted to be fixedly carried by the machine and a flexure assembly carried by the support member. The flexure assembly is in the form of at least one flat spring including connections along a central portion. The central portion is fixedly mounted to the support member, and an outer peripheral portion of the flat spring provides at least in part a movable counterbalance mass. The flexure assembly presents the counterbalance mass for movement in substantial alignment with a desired rectilinear component of vibration of the machine to counterbalance vibrations emanating therefrom. A vibration balanced machine having the passive balance system is also disclosed.
WO 2014/206542 describes a compensating oscillation device for a linear piston system. The compensating oscillation device comprises a housing, at least two coupling elements, and a centrifugal mass, which can be deflected along an axis by means of each coupling element and which is coupled to the housing. Each coupling element is fastened to the housing at least one fastening region and to the centrifugal mass at least one connecting region. According to the invention, on each of the at least two coupling elements, the at least one connecting region to the centrifugal mass is radially closer to the axis than the at least one fastening region to the housing, and the centrifugal mass is arranged between two coupling elements in the axial direction in the idle state. The invention further relates to a linear piston system, comprising a piston, which is supported in such a way that the piston can be moved linearly, and comprising such a compensating oscillation device.
US 2009/007560 describes a vibration suppression apparatus includes a leaf spring having one end connected to one end in the vibrating direction of a Stirling refrigerator which is a reciprocating motion apparatus, a balance mass connected to the other end of the leaf spring, and a damper including a damping body connected to the balance mass and vibrating in phase with the balance mass. With this structure, the high-performance vibration suppression apparatus including the elastic body and the damper can be manufactured with a small size and at a low cost.
The disadvantage of such a single degree of freedom (SDOF) TDAs is that their design allows motion of the counterbalance mass in essentially axial direction, thus, in case of side-by-side package, the only compressor induced force export may be attenuated. For vibration sensitive applications relying on side-by-side packaged cryocooler, the secondary, smaller and matched TDA may be mounted inline with the expander. This results in added mechanical complexity and extra cost.
General Description
There is, thus, a need in the art for a novel vibration attenuation technique suitable for use in association with mechanical system, e.g. comprising two or more active components, generating corresponding two or more parallel cyclically varying mechanical forces. The present invention provides a system and technique for attenuating vibrations generated as a result of spaced apart parallel forces having essentially common frequency with certain phase lag between the forces. In this connection the technique of the invention is generally described herein with respect to split Stirling type cryogenic refrigerator, however it should be understood broadly and suitable for attenuating vibrations generated by various other mechanical forces.
To this end the vibration attenuation unit of the invention is generally connectable with mechanical system, such as split Stirling cryogenic refrigerator, via a rigid connection and configured to reduce vibration of the mechanical system. The vibration attenuation unit is typically configured to attenuate vibration at driving frequency of the mechanical system. Further, the vibration are typically associated with two (translation and tilt) or more, generally three (translation and two tilts) modes of vibrations. In some configurations, additional vibration modes may also be attenuated based on configuration of the vibration attenuation unit.
The technique of the invention is based on the inventor's understanding that by matching the resonant frequency of a “mass-spring” tuned dynamic absorber (TDA) with typical driving frequency, the reaction force produced by the TDA is equal in magnitude and opposite in direction to the vibration export. Therefore, the use of appropriately tuned TDA eliminates, or at least significantly reduces the residual vibration. Further, a single mass TDA may be configured as having substantially similar resonant translational and tilt frequencies, thereby extending the aggregate effect.
As indicated above, the present invention is described herein in accordance with the exemplary configuration of a split Stirling cryogenic refrigerator. A split Stirling refrigerator is generally configured with a side-by-side mounted compressor and expander units. In general, vibration export produced by such a split linear cooler may be thought of as a pair of parallel and coherent dynamic forces acting along the compressor and expander axis or as a resultant dynamic force and moment, the frequency of which equals the driving frequency. Depending on the payload inertia and mounting conditions, relative distance between the compressor and expander units along with the distance from the payload center of gravity, this vibration export may result in translation and angular vibration manifesting itself in the form of dynamic defocusing occurring when translational displacement becomes comparable with the focus depth and line of sight jitter occurring when the amplitude of in-plane focal plane array (FPA) motion becomes comparable with size of pixel.
The reciprocating motion of a compressor piston provides the cyclic pressure and the volumetric flow of a working agent (helium, typically) in the expansion space of an expander. A displacer, reciprocating inside a cold finger, shuttles the working agent back and forth from the cold side to the warm side of the cooler. During the expansion stage of the thermodynamic cycle, heat is absorbed from the IR detector mounted upon the cold finger tip (cold side of a cycle), and during the compression stage, heat is rejected to the ambient from the cold finger base (warm side of a cycle).
Thus, the present invention, in some configurations thereof, provides a closed cycle split Stirling cryocooler, utilizing side-by-side configuration of the compressor and expander units (e.g. with particular minimum offset), whereupon the cooler induced translational and angular vibrations are attenuated down to the acceptable levels using a vibration attenuation unit, e.g. including tunable multimodal tuned dynamic absorber (TDA). The vibration attenuation unit is configured to eliminate, or at least significantly attenuate, cooler induced vibration. In the exemplary configuration of a side-by-side split Stirling cryogenic refrigerator, these typical vibration modes generally include one translational mode along axis of translation of the compressor piston and two tilting modes. Additionally, the frequencies of the different vibration modes are essentially similar and equal the driving frequency.
The vibration attenuation unit according to some embodiments of the present invention utilizes a multimodal TDA comprising at least one planar flexural bearing and a proof mass assembly. The flexural bearing is configured as a spring-like planar element, rigidly connected to the mechanical system (e.g. Stirling cryocooler) at one end thereof and to the proof mass assembly at another end. In some configuration, the flexural bearing may be configured in the form of a circular planar spring with symmetrical spiral slots. Further, the proof mass assembly may be formed by coaxial arrangement comprising at least primary and secondary circular proof mass elements (typically in the form of rings). The circular spring element may be connected to the mechanical system at a central anchor thereof and the proof mass assembly may be mounted in a circular symmetric fashion at periphery of the circular spring.
Further, in some embodiments, the central and periphery portions of the flexural bearing are configured with frictionless (substantially rigid) features configured to provide fixed and rigid fastening to the mechanical system (e.g. through the compressor housing of a split Stirling refrigerator) and to the proof mass assembly, respectively. Generally, according to some embodiments, the secondary proof ring of the proof mass assembly may be configured to be displaceable along an axial direction with respect to the primary proof ring. Thus, in these embodiments, the primary proof ring is attached to the flexural bearing in a fixed location and the secondary proof ring may be fastened at a selected location. For example, the secondary proof rind may be configured to slide axially with respect to the primary proof ring and fastened in selected appropriate position using one or more radial screws (e.g. setscrews).
In such embodiments, the resonant translational frequency of the vibration attenuation system is dependent on the total mass of the proof mass assembly and axial spring rate of the flexural bearing. Additionally, resonant frequencies of the vibration attenuation unit along tilt modes depend on the angular spring rate of the said flexural bearing and moment of inertia of the proof mass assembly. The moment of inertia of the proof mass assembly can be tuned by selection of axial displacement of the secondary proof ring along the axial direction. Thus, by mechanical design of the flexural bearing and proof rings, the frequencies of the translation and tilting modes may be configured to be essentially equal the working frequency typical of the associated mechanical system.
More specifically, the total mass of the proof mass assembly may be determined in accordance with the spring rate of the flexural bearing to provide the desired axial frequency. Further, location of the secondary proof ring is selected to tune moment of inertia of the vibration attenuation unit in accordance with the spring rate of the flexural bearing and provide substantially similar frequency without affecting the aggregate mass and, therefore, translational frequency. In this connection it should be noted that the term substantially similar frequency relates to a frequency that is equal or almost equal to frequency of operation of the mechanical system. This enables tuning of operational frequency of the vibration attenuation unit in accordance with typical frequency of the associated mechanical system, e.g. driving frequency of a split Stirling cryogenic refrigerator.
Thus, according to a broad aspect of the invention, there is provided a cryogenic refrigerator system comprising: linear Split Stirling unit having an expander unit and a compressor unit mounted in a side by side configuration upon a common frame, and a vibration attenuation unit attached to the Split Stirling unit. Wherein said vibration attenuation unit is configured for attenuation of two or more modes of vibration characterized by a frequency corresponding to operation frequency of said linear Split Stirling unit. Generally, according to some embodiments, the operation frequency of the Split Stirling unit may be a fixed frequency.
According to some embodiments, the vibration attenuation unit may be configured for vibration attenuation along at least one axial mode and at least two tilt modes with respect to a predetermined reference axis of the system.
According to some embodiments, the vibration attenuation unit may comprise an undamped mass-spring system comprising a planar flexural bearing and a proof mass assembly. The planar flexural bearing may be configured as a planar circular disc comprising a plurality of symmetrical spiral slots, said planar flexural bearing is connectable to the proof mass assembly at a peripheral anchor and to said Split Stirling unit at a central anchor thereof. The flexural bearing may typically be made of metal or metal alloy. Generally the material of the flexural bearing may have spring features/properties.
According to some embodiments, total mass and arrangement of the proof mass elements of the proof mass assembly are configured with respect to corresponding spring constants of said flexural bearing to provide desired resonant frequencies associated with said of two or more modes of vibration. The desired resonant frequencies are preferably selected to be substantially similar to an operational frequency of said linear Split Stirling unit. To this end the term substantially related to a reasonable margin of error corresponding with acceptable variation in driving frequency of the split Stirling unit, which may be within a range of up to 0.1 Hz.
According to some embodiments, the vibration attenuation unit is configured with a predetermined mass of the proof mass assembly and predetermined axial and angular spring constants. The total mass of the proof mass assembly may generally be determined to provide a desired axial frequency for minimizing axial vibrations. When the vibration attenuation unit is attached to a mechanical system, e.g. the split Stirling cryogenic refrigerator, the driving frequency of the refrigerator can be tuned to determine working frequency providing minimal axial vibration amplitude.
According to some embodiments, the proof mass assembly may comprise at least one primary proof mass element mounted fixedly on a peripheral anchor of said flexural bearing, and at least one secondary proof mass element mounted on and moveable with respect to said primary proof mass element. The primary and secondary proof mass elements may preferably be configured as concentric rings.
Generally, according to some embodiments, the vibration attenuation unit is configured to be circularly symmetric.
In some configurations of the cryogenic refrigerator, the vibration attenuation unit may be mounted in-line with axis of translation of a piston of said compressor unit.
Additionally, according to some embodiments of the inventions, the vibration attenuation unit may be located within an evacuated chamber having sub-atmospheric pressure for reducing the aerodynamic damping, aural noise generation and transmission. The pressure within the evacuated chamber may be in the range of 10−2 to 10−4 Torr.
According to yet another broad aspect of the invention there is provided a system comprising a mechanical system comprising two or more axial moving elements generating two or more cyclic forces along parallel axes, said two or more cyclic forces having common frequency and certain phase difference between them, and a vibration attenuation unit. The vibration attenuation unit comprises flexural bearing connectable to said mechanical system and to a proof mass assembly, wherein total mass and arrangement of masses of said proof mass assembly are configured to provide resonant frequencies corresponding to two or more modes of vibrations to be similar to said common frequency of the cyclic forces of said mechanical system, thereby providing vibration attenuation along said two or more modes of vibrations.
The mechanical system may generate cyclic axial vibrations along an axis parallel to axes of said cyclic forces and cyclic tilt vibrations about certain point of reference, the vibration attenuation unit may thus be configured with resonant frequencies for vibrations mode corresponding with said cyclic axial vibrations and said cyclic tilt vibrations to thereby attenuate said axial and tilt vibrations.
According to some embodiments of the invention, the flexural bearing of said vibration attenuation unit may be configured as a flat spring element and is attached to said mechanical system at a central region thereof. Additionally, the proof mass assembly may be mounted at periphery of said flat spring and may comprise first fixed proof mass element and second proof mass element moveable along a predetermined axis; wherein variation in location of said second proof mass enables tuning of resonant frequency of the vibration attenuation unit with respect to at least one vibration mode while not affecting resonant frequency with respect to at least one other vibration mode, thereby enabling tuning of anti-resonant frequencies with respect to said two or more modes of vibrations to be substantially similar.